The present invention relates generally to systems and methods for navigating mobile platforms, and particularly to a beacon-based positioning or navigation system utilizing two-way ultrasonic ranging.
The use of ultrasonic ranging in air to position or navigate a mobile object with respect to a set of fixed beacons is widespread nowadays. Many of these systems use time-of-flight measurements of an ultrasonic signal from a transmitter to a receiver. The transmitter can either be located at the mobile object to be positioned, with receivers on beacons placed strategically in the environment, or conversely, transmitters can be placed on the beacons with a receiver located on the mobile object. In some systems, the ultrasonic signal is accompanied by a radio frequency signal, mainly for the purpose of time-synchronization between the transmitter and the receiver. In some other systems, the difference in flight times of an ultrasonic signal and one other signal such as a radio frequency signal is used to determine the distance between the mobile object and each of the beacons. In situations where the mobile object is moving with a significant speed with respect to the beacons, signal frequency Doppler shift of the transmitted ultrasonic signal may be used to aid in the determination of the velocity and position of the mobile object.
In these conventional systems, the distance-measuring accuracy is dependent on the correct determination of the speed of propagation of the ultrasonic signal, i.e., the speed of sound in air. The sound speed in air is dependent on many factors, such as atmospheric temperature, humidity, and wind effect. The use of currently available thermometers and hygrometers makes it very difficult to measure the temperature and humidity along the entire wave path and hence predict the speed of sound along the entire path of an ultrasonic signal. Further, in the presence of a wind current, existing systems are subject to large errors in the calculation of distance and hence large positioning errors. The present invention overcomes these problems of prior art ultrasonic ranging systems.
Additionally, most conventional navigation systems require that the system installer survey the locations of the beacons manually and input the locations into the system. See E. LeMaster and S. Rock, Self-Calibration of Pseudolite Arrays Using Self-Differencing Transceivers, Institute of Navigation GPS-99 Conference, Nashville, Tenn., September 1999. Some systems have been classified as self-calibrating, but still require the user to perform certain precise distance measurements. See Mahajan, A. and Figueroa, F., An Automatic Self Installation and Calibration Method for a 3-D Position Sensing System using Ultrasonics, Japan-USA Symposium on Flexible Automation, Boston, Jul. 7-10, 1996, pp. 453-460. The present invention also avoids the beacon surveying requirements of prior art systems.
The system and method of the present invention overcome the aforementioned problems with two-way ultrasonic positioning and navigation that mitigates wind-induced changes in speed of sound. Moreover, the two-way ultrasonic ranging technique obviates the need for time synchronization between platforms. In one embodiment of the present invention, an autonomous or semi-autonomous apparatus, or a mobile platform, is positioned or navigated with respect to a few beacons having fixed positions. Positioning applications involve the localization of the mobile platform using two-way ultrasonic time-of-flight measurements. Navigation applications involve tracking the mobile platform using augmenting sensors based on the position of the mobile platform determined through the two-way ultrasonic time-of-flight measurements.
The method for a two-way ultrasonic measurement, according to one embodiment of the present invention, comprises transmitting an initiating ultrasonic signal from an initiator, the initiating ultrasonic signal identifying an intended recipient, receiving the initiating ultrasonic signal at the intended recipient, transmitting a responding ultrasonic signal from the recipient after a predetermined time delay from the reception of the initiating ultrasonic signal, receiving the responding ultrasonic signal at the initiator, and determining a distance between the initiator and the respondent based on a time period starting at the transmission of the initiating ultrasonic signal and ending at the reception of the responding ultrasonic signal, and on knowledge about the predetermined time delay and the length(s) of the initiating and responding ultrasonic signals. The initiator may be associated with the mobile platform and the respondent with one of the beacons, or vice versa, or the originator and recipient may both be beacons.
A system according to one embodiment of the present invention includes a first set of modules preferably placed on the mobile platform and a second set of modules preferably placed on each of the beacons. The two sets of modules are communicatively coupled through ultrasonic signals and in some embodiments by RF signals also. Each set of modules includes an ultrasonic array coupled to a digital signal processor through an analog/digital converter. The ultrasonic array further includes at least one ultrasonic transceiver or transducer pair for transmitting and receiving ultrasonic signals. The digital signal processor controls the transmission and reception of the ultrasonic signals, including calculating signal arrival time and a figure-of-merit on the quality of a received ultrasonic signal. The first set of modules further includes a central processing unit coupled to the digital signal processor for implementing a position algorithm for determining the position of the mobile platform based on the two-way ultrasonic time-of-flight measurements.
The system can be self-calibrating to eliminate the need for manual survey equipment for determining the positions of the beacons. A method for self-calibrating the locations of the plurality of beacons comprises the steps of: (1) determining a set of inter-beacon distances using inter-beacon ultrasonic measurements; (2) setting a first beacon at an origin of a coordinate system; (3) setting a second beacon on one axis of the coordinate system and determining the coordinates of the second beacon based on the distance between the first beacon and the second beacon; (4) determining the coordinates of a third beacon based on the distances between the first beacon and the second beacon, between the second beacon and the third beacon, and between the third beacon and the first beacon, and based on a predetermined constraint on the location of the third beacon; and (5) iteratively improving the determined locations of the plurality of beacons.